Fuel cell system and its startup control

ABSTRACT

Before warmup of a fuel cell stack ( 1 ) is complete, a cooling water pressure of the fuel cell stack ( 1 ) is suppressed lower than the pressure used when the fuel cell system is run in the steady state. In this way, when the system is started from a low temperature state, water in a cathode ( 2 ) and anode ( 3 ) flows efficiently into the cooling water passage ( 9 ), and water clogging is prevented while maintaining a proper water balance in the fuel cell system.

FIELD OF THE INVENTION

This invention relates to a fuel cell system, and more particularly tostartup control when a fuel cell system is started from a lowtemperature state.

BACKGROUND OF THE INVENTION

A fuel cell system transforms the energy of fuel, directly intoelectrical energy. In the fuel cell system disclosed by JP8-106914Apublished by the Japanese Patent Office in 1996, in a pair of electrodesprovided on either side of a membrane electrode, fuel gas containinghydrogen is supplied to the anode and an oxidizing agent gas containingoxygen is supplied to the cathode. Electrical energy is then extractedfrom the electrodes, using the electrochemical reactions produced on thesurface of the membrane electrode, i.e.,

-   -   anode reaction: H₂→2H⁺+2e⁻    -   cathode reaction: 2H⁺+2e⁻+(½)O₂→H₂O

The fuel gas supplied to the anode, may be supplied directly from ahydrogen storage device, or fuel containing hydrogen may be reformed,and the reformed hydrogen-containing gas supplied to the anode. The fuelcontaining hydrogen may be natural gas, methanol or gasoline, and theoxidizing agent gas supplied to the cathode is generally air.

As it is necessary to keep the humidification state of the membraneelectrode optimal in order to extract the performance of the membraneelectrode in the fuel cell and enhance the power generation efficiency,the fuel gas and air introduced into the fuel cell is humidified. Whenreforming fuel gas and extracting hydrogen as mentioned above, water isused for reforming. Therefore, in order to use the fuel cell forvehicles, a water balance must be maintained within the fuel cell systemincluding the fuel cell or reforming device. This is because thepracticality of a fuel cell vehicle will fall remarkably if water runsshort and it becomes necessary to supply pure water periodically.

There are two means of supplying water to the membrane electrode. One isthe method of humidifying the fuel gas or air with a humidifier, andhumidifying the membrane electrode using the moisture. Another is themethod of connecting a cooling water passage to the anode and cathode inthe fuel cell via a bipolar plate comprising a porous material, andsupplying water to the membrane electrode from the cooling water tohumidify the membrane electrode. Since the latter method does not need ahumidifier for fuel gas and air, there is an advantage in that thesystem construction is simpler.

SUMMARY OF THE INVENTION

When starting the fuel cell system at low temperature, the watergenerated inside the fuel cell and the moisture in the fuel and aircondense inside the fuel cell, and may block the fuel and air passage,which is called “water clogging.” Water clogging reduces the efficiencyof the fuel cell, and in particular, water clogging which occurs duringstartup lengthens the warm-up time of the fuel cell. As the frequency ofstartup operations is high in vehicles, the lengthening of the warm-uptime of the fuel cell system reduces the usability of the fuel cellvehicle.

One of the methods of resolving water clogging is the blowing away ofthe condensed water by increasing the pressure of the fuel gas or air.However, if gas or air is delivered to the fuel cell at higher thanordinary operating pressure, the membrane electrode inside the fuel cellor the durability of the sealing will be degraded, the performance ofthe fuel cell will be reduced and its life will be shortened. In thefuel cell for vehicles, although there are differences in the frequencyof use, startup operations may be performed from hundreds of times tothousands of times, so the above decline of performance is remarkable.Also, the ability to vary the pressure of fuel gas and air makes thesystem construction complicated.

Another method is to heat the fuel cell stack itself. However, theheating of the fuel cell stack makes the system complex. If the energyrequired for heating becomes large, the fuel economy of the vehicledecreases. Further, the time required for heating is too long for thevehicle.

It is therefore an object of this invention to provide a fuel cell stackwherein a cooling water passage is connected to a fuel gas passage andair passage via a porous plate, wherein the water in the anode andcathode flow out efficiently to the cooling water passage during startupfrom low temperature, the water balance in the fuel cell stack ismaintained, and the problem of clogging is resolved.

In order to achieve above object, this invention provides a fuel cellsystem, comprising a fuel cell stack wherein a cooling water passage andan electrode are connected via a porous plate through which water canpass, a pressure adjusting device which adjusts the pressure of coolingwater in the cooling water passage, and a controller. The controllerfunctions to determine whether warmup of the fuel cell stack is completebased on the running state of the fuel cell stack, and before warmup ofthe fuel cell stack is complete, control the pressure adjusting deviceto decrease the cooling water pressure lower than the cooling waterpressure after warmup is complete.

According to an aspect of this invention, this invention provides astartup method for a fuel cell system including a fuel cell stackwherein a cooling water passage and an electrode are connected via aporous plate through which water can pass and a pressure adjustingdevice which adjusts the pressure of cooling water in the cooling waterpassage, the method comprising determining whether warmup of the fuelcell stack is complete based on the running state of the fuel cellstack, and before warmup of the fuel cell stack is complete, controllingthe pressure adjusting device to decrease the cooling water pressurelower than the cooling water pressure after warmup is complete.

The details as well as other features and advantages of this inventionare set forth in the remainder of the specification and are shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a fuel cell system relating to thisinvention.

FIG. 2 is a flowchart showing the startup processing of the fuel cellsystem.

FIG. 3 is a flowchart showing the startup processing taking thehumidification state of a membrane electrode into consideration.

FIG. 4 is similar to FIG. 1, but is a block diagram of a fuel cellsystem according to the second embodiment.

FIG. 5 is a flowchart showing the startup processing according to thesecond embodiment.

FIG. 6 is similar to FIG. 1, but is a block diagram of a fuel cellsystem according to the third embodiment.

FIG. 7 is a flowchart showing the startup processing according to thethird embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, a fuel cell system relating to thisinvention is provided with a fuel cell stack 1 comprising plural cellsc1, c2, . . . , cn, pressure sensors 5, 6, 7, a cooling water pump 12, acooling water tank 13, a heat exchanger 14 and a controller 16. Atemperature sensor 15 for measuring the internal temperature Tsin of thefuel cell stack 1 is installed inside the fuel cell stack 1. Measurementsignals from these sensors are input into the controller 16.

Pressure sensors 5, 6, 7 measure the pressure in an air passage 8 whichsupplies air, a cooling water passage 9 which supplies cooling water anda fuel gas passage 10 which supplies fuel gas to the fuel cell stack 1,in the vicinity of the inlet to the fuel cell stack 1.

The controller 16 comprises one, two or more microprocessors, a memoryand an input/output interface. The controller 16 calculates a pressuredifference ΔP between the cooling water passage 9 and electrodes, acathode 2 and anode 3 at the inlet of the fuel cell stack 1, from thesignals from the pressure sensors 5, 6, 7. When the system is started,the controller 16 determines a pressure Pcw of cooling water accordingto the internal temperature Tsin of the fuel cell stack 1 detected bythe temperature sensor 15, and controls the opening of a pressurereducing valve 11 and the rotation speed of the cooling water pump 12 sothat a determined cooling water pressure Pcw is realized.

Specifically, before warm-up of the fuel cell stack 1 is complete, whenthe internal temperature Tsin of the fuel cell stack 1 is low and whenwater clogging by water condensed in the cathode 2 and anode 3 mayoccur, the pressure Pcw of cooling water is controlled to be lower thanthe pressure Pnormal used in the steady state after warmup is complete.In this way, the water condensed in the cathode 2 and anode 3 can bemade to flow into the cooling water passage 9 efficiently using thepressure difference via the holes in the porous plate 4, and waterclogging can be prevented. The cooling performance of the fuel cellstack 1 falls due to the lowering of the cooling water pressure Pcw.However, as this happens before warmup of the fuel cell stack 1 iscompleted, this does not pose a problem. Conversely, cooling of the fuelcell stack 1 can be suppressed and warmup can be promoted. As water doesnot flow out to the outside of the system from the air passage 8 and thefuel gas passage 10, the total amount of the water in the fuel cellstack 1 is kept constant, i.e., the water balance is maintained.

Subsequently, if the internal temperature Tsin of the fuel cell stack 1rises and warmup is completed, water clogging cannot occur, so thepressure Pcw of cooling water is controlled to the pressure Pnormal inthe steady state, and the cooling performance of the fuel cell stack 1is ensured.

FIG. 2 is a flowchart showing startup processing, and this is performedat a predetermined time, for example every 10 milliseconds, by thecontroller 16.

First, in a step S11, fuel gas, air and cooling water start to beintroduced to the fuel cell stack 1.

In a step S12, the internal temperature Tsin of the fuel cell stack 1 ismeasured. In a step S13, the measured internal temperature Tsin and thepredetermined temperature Tth are compared. The predeterminedtemperature Tth is set according to the shape of the gas, air andcooling water passages, and the construction of the system. For example,it is set to the lowest temperature of the temperatures at which waterclogging by the water condensed in the cathode 2 and anode 3 of the fuelcell stack 1 does not occur. The predetermined temperature Tth is setlower than the running temperature (about 60° C.–70° C.) of the fuelcell stack 1. From experiment, it was found that water clogging does notoccur above 40° C., and it was possible to run the vehicle by settingthe cooling water pressure Pcw to the pressure Pnormal, so thepredetermined temperature Tth was set to 40° C.

It was determined that when the internal temperature Tsin is lower thanthe predetermined temperature Tth, water clogging occurs due to thewater condensed in the cathode 2 and anode 3, so the routine proceeds tostep S14, the water pressure Pcw is controlled to a predetermined lowpressure power Plow, and the water condensed in the cathode 2 and anode3 is made to flow out to the cooling water passage 9.

In order to increase the outflow efficiency of the condensed water, itis preferred to make the cooling water pressure Pcw as low as possiblerelative to the cathode 2 and anode 3. However, if the differentialpressure ΔP between the cooling water passage 9 and the electrodes 2, 3(cathode 2 and anode 3) becomes too large, fuel gas or oxidizing agentgas penetrates the cooling water passage 9. If the differential pressureΔP is further increased, the internal structure of the membraneelectrode or cell may break down. Thus, although the cooling waterpressure Pcw is controlled to be as low as possible, the differentialpressure ΔP between the cooling water passage 9 and the electrodes 2, 3is controlled so that the pressure is higher than the minimum value ofthe pressure at which fuel gas or oxidizing agent gas penetrates thecooling water passage 9.

While the internal temperature Tsin of the fuel cell stack 1 is lowerthan the predetermined temperature Tth, the steps S12, S13, S14 arerepeated. Subsequently, if warmup of the fuel cell stack 1 proceeds andthe internal temperature Tsin exceeds the predetermined temperature Tth,it is determined that the condensation amount of the water in the fuelcell stack 1 is sufficiently small, water clogging by the condensedwater does not occur, and the routine proceeds to a step S15.

When the routine proceeds to the step S15, the cooling water pressurePcw is controlled to the pressure Pnormal normally used in steadyrunning. Thereafter, the cooling water pressure Pcw is controlled tothis ordinary pressure Pnormal, and the cooling performance of the fuelcell stack 1 is ensured.

Here, the differential pressure ΔP between the cooling water passage 9and the electrodes 2, 3 (cathode 2 and anode 3) is calculated, and theminimum of the cooling water pressure Pcw is set based thereon. However,the pressure value for low cooling water pressure operation (fixedvalue) may be predefined, and the cooling water pressure Pcw changedover to this preset value immediately in the step S14 when low coolingwater pressure running is performed. Thus, the system construction canbe simplified and the control response can be improved.

As water may freeze when the internal temperature Tsin of the fuel cellstack 1 is lower than 0° C., when the detected internal temperature Tsinis lower than 0° C., a routine not shown is performed, and the internalwater must be thawed by heating with a heater or circulating hightemperature gas to the passage in the fuel cell stack 1 beforehand, andthe aforesaid processing performed afterwards.

Moreover, as it is necessary to appropriately humidify the membraneelectrode of the fuel cell stack 1, the humidification state of themembrane electrode is detected, and when the water content of themembrane electrode is smaller than a predetermined value, the pressurePcw of the cooling water passage 9 is controlled to a second lowpressure Plow2 (Plow<Plow2<Pnormal) which is lower than the normalrunning pressure Pnormal and higher than the pressure Plow set in thestep S14. By adding such processing, even the movement of the moisturein the membrane electrode to the cooling water passage 9 can beprevented, and the humidification state of the membrane electrode can bemaintained at a satisfactory level.

FIG. 3 is a flowchart showing the startup processing when taking thehumidification state of the membrane electrode into consideration. StepsS16–S18 are added to what was shown in FIG. 2.

The humidification state of the membrane electrode is detected in thestep S16. The humidification state of the membrane electrode can bepredicted from the state change of the fuel cell stack 1 such as thechange of electromotive force and the reserved water amount in the watertank 13. More specifically, to predict this more directly, thetemperature is detected for every cell forming the fuel cell stack 1,the temperature distribution of cells is measured as shown in theembodiment described below, and the prediction is based thereon.

When it is determined in the step S17 that humidification of themembrane electrode is not sufficient, the routine proceeds to the stepS18, and the pressure Pcw of the cooling water passage 9 is controlledto the pressure Plow2 which is lower than the normal running pressurePnormal and higher than the pressure set in the step S14.

Next, a second embodiment of this invention will be described.

FIG. 4 shows the construction of a fuel cell system of the secondembodiment. Temperature sensors 21 are installed which measure thetemperatures in each cell c1, c2, . . . , cn of the fuel cell stack 1,which is a difference from the first embodiment. For convenience, onlythree sensors 21 have been shown in FIG. 3, but there may be a larger orsmaller number of the sensors 21 according to the number of the cells.

Moreover, the startup processings of the system performed by thecontroller 16 is also different. The controller 16 predicts waterclogging from the temperature distribution of the cells, and performspressure control of cooling water. The average of the cell temperaturesis calculated, and the cooling water pressure Pcw is also controlledbased on the differential pressure ΔP between the cooling water and theelectrodes 2, 3 (cathode 2 and anode 3) at the inlet of the fuel cellstack, and the average Tcelave of cell temperature.

Specifically, the controller 16 predicts water clogging from thedispersion (bias) in the distribution of the cell temperatures. Forexample, there is a bias in a temperature distribution such as when thetemperature of some cells is low and the possibility of water cloggingis predicted, the pressure Pcw of cooling water is immediately reducedto the minimum valve Plow of pressure at which penetration of fuel gasor oxidizing agent gas into the cooling water passage 9 does not takeplace, and the outflow efficiency of water from the cathode 2 and anode3 to the cooling water passage 9 is increased to the maximum so as torapidly eliminate any water clogging.

Even after water clogging has been resolved, before warmup of the fuelcell stack 1 is complete, when the average temperature Tcelave of thecell is lower than the predetermined temperature Tth and there is apossibility that water clogging may occur in the fuel cell stack 1, thepressure Pcw of cooling water is controlled lower than the pressurePnormal in steady running as in the preceding embodiment, and water inthe cathode 2 and anode 3 is made to flow efficiently into the coolingwater passage 9 via the holes in the porous plate 4.

FIG. 5 is a flowchart showing startup processing of the fuel cellsystem, and is performed at a predetermined time, for example every 10milliseconds, by the controller 16.

First, in a step S21, introduction of fuel gas, air and cooling water tothe fuel cell stack 1 is started. In a step S22, the temperatures Tcel1,Tcel2, . . . , Tceln of the cells c1, c2, . . . , cn of the fuel cellstack 1, are measured.

In a step S23, water clogging is predicted from the distribution of celltemperature. When the temperature of only some cells is lower than thetemperature of other cells, it is predicted that water clogging mayoccur in the cells at low temperature. When it is predicted that thereis water clogging, the routine proceeds to a step S24, and the coolingwater pressure Pcw is set to the minimum pressure Plow. The minimumpressure Plow is the minimum value of pressure at which penetration offuel gas or oxidizing agent gas into the cooling water passage 9 doesnot take place, and is set based on the differential pressure ΔP betweenthe cooling water passage 9 and the electrodes 2, 3 as in the precedingembodiment. By lowering the cooling water pressure Pcw to the minimumpressure Plow, the outflow of condensed water in the cathode 2 and anode3 to the cooling water passage 9 is enhanced to the maximum, and waterclogging can be eliminated at an early stage.

When it is determined that the bias of temperature distribution hasdisappeared and water clogging in the fuel cell stack 1 was eliminated,the routine proceeds to a step S25 and the average temperature Tcelaveof the cells is calculated.

In a step S26, the average temperature Tcelave of the cells is comparedwith the predetermined temperature Tth (the minimum temperature of thetemperatures at which water clogging due to the condensed water does notoccur), and when the cell average temperature Tcelave is lower than thepredetermined temperature Tth, the routine proceeds to a step S27, andthe cooling water pressure Pcw is set lower than the pressure for steadyrunning, Pnormal, as in the step S14 of the preceding embodiment. Whenthe cell average temperature Tcelave subsequently reaches thepredetermined temperature Tth, the routine proceeds to a step S28, andthe cooling water pressure Pcw is set to the pressure used for steadyrunning, Pnormal.

In this embodiment, the cell temperature is measured, and the presenceor absence of water clogging is determined from the temperaturedistribution. When it is determined that there is water clogging, thecooling water pressure Pcw is lowered to the minimum pressure Plow, andpriority is given to the outflow of water from the cathode 2 and anode 3into the cooling water passage 9. Thereby, water clogging in the fuelcell stack 1 can be eliminated at an early stage. After water cloggingis eliminated, the same startup processing can be performed as in thepreceding embodiment according to the cell average temperature Tcelave,and thereafter, the fuel cell system can be started without waterclogging.

Also in this embodiment, as in the first embodiment, the humidificationstate of the membrane electrode of the fuel cell stack 1 is predictedfrom the electromotive force etc., and if the humidification of themembrane electrode is not sufficient, the cooling water pressure Pcw israised to Plow2 which is higher than the pressure Plow set in the stepS27, movement of water into the cooling water passage 9 from the cathode2 and anode 3 is suppressed, and humidification of the membraneelectrode is assisted as in the first embodiment. In particular, in thisembodiment, as the cell temperature distribution is detected, thehumidification state of the membrane electrode can be predicted withhigh precision from the change of cell temperature distribution andelectromotive force.

Next, a third embodiment of this invention will be described.

FIG. 6 shows the construction of the fuel cell system of the thirdembodiment. This is essentially identical to the construction of thefirst embodiment, however temperature sensors 31, 32 are formed in thecooling water passage 9 near the inlet and outlet of the fuel cell stack1, respectively. Also, the startup processing of the system performed bythe controller 16 is also different.

The controller 16 calculates a temperature difference ΔT of the coolingwater between the fuel cell inlet and outlet. From this temperaturedifference ΔT, the reaction status inside the fuel cell stack 1 can begrasped, water clogging can be estimated and cooling water pressurecontrol performed. The temperature difference ΔT can be measured byinstalling temperature sensors 31, 32 at the fuel cell inlet and outlet,respectively, as shown in FIG. 6, or by installing a thermocouple in thecooling water passage 9. In both cases, temperature measurement can beperformed more easily than by measuring the internal temperature of thefuel cell stack 1 by installing sensors inside the fuel cell stack 1.

FIG. 7 is a flowchart showing startup processing of the fuel cellsystem, and is performed at a predetermined time, for example every 10milliseconds, by the controller 16.

First, in a step S31, introduction of fuel gas, air and cooling water tothe fuel cell stack 1 is started.

In a step S32, the cooling water temperatures Tin, Tout at the inlet andoutlet of the fuel cell stack 1 are measured. In a step S33, thetemperature difference ΔT of cooling water between the inlet and outletof the fuel cell stack 1 is computed.

In a step S34, this temperature difference ΔT and a preset temperaturedifference ΔTth are compared. For example, if the outside atmospherictemperature is approximately 20° C. and the running temperature of thefuel cell stack 1 is approximately 60° C.–70° C., the temperaturedifference ΔTth is set to a value between 20° C.–50° C. When thetemperature difference ΔT is smaller than ΔTth, it is determined thatwarmup of the fuel cell stack 1 is not complete and water clogging dueto condensation of water may occur in the fuel cell stack 1, so theroutine proceeds to a step S35, and the cooling water pressure Pcw isset to Plow (<Pnormal).

Subsequently, if warmup of the fuel cell stack 1 continues and thetemperature difference ΔT becomes larger than ΔTth, it is assumed thatwarmup of the fuel cell stack 1 is complete, and the routine proceeds toa step S36. Thereafter, the cooling water pressure Pcw is controlled tothe pressure Pnormal normally used for steady running.

Even if warmup completion of the fuel cell stack 1 is determined basedon the temperature difference ΔT of cooling water at the inlet andoutlet of the fuel cell stack 1, the same startup processing as in thepreceding embodiment is possible.

Although the warmup state of the fuel cell stack 1 was determined herebased on temperature difference ΔT of cooling water at the inlet andoutlet of the fuel cell stack 1, the warmup state of the fuel cell stack1 may be determined based on the temperature difference of fuel gas orair at the inlet and outlet of the fuel cell stack 1.

Further, the sensor at the inlet may be omitted, and the warmup state ofthe fuel cell stack 1 may be determined from the temperature rise offluid (cooling water, fuel gas, or air) at the outlet. For example, ifit is determined that warmup of the fuel cell stack 1 is complete whenthe temperature of cooling water at the outlet has risen to apredetermined temperature, the system construction is simplified andcontrol is also simplified.

Moreover, also in this embodiment, the humidification state of themembrane electrode of the fuel cell stack 1 may be detected from theelectromotive force etc., as in the preceding embodiment, and if thehumidification of the membrane electrode is not sufficient, the coolingwater pressure is raised to Plow2 which is higher than the pressure Plowset in the step S35, movement of water into the cooling water passage 9from the cathode 2 and anode 3 is suppressed, and humidification of themembrane electrode is assisted.

The entire contents of Japanese Patent Application P2001-342937 (filedNov. 8, 2001) are incorporated herein by reference.

Although the invention has been described above by reference to acertain embodiment of the invention, the invention is not limited to theembodiment described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inthe light of the above teachings. The scope of the invention is definedwith reference to the following claims.

INDUSTRIAL FIELD OF APPLICATION

This invention may be applied to various fuel cell systems includingthose of vehicles, and is useful for preventing water clogging whenstarting from a low temperature state, and enhancing startup of the fuelcell system.

1. A fuel cell system, comprising: a fuel cell stack wherein a coolingwater passage and an electrode are connected via a porous plate throughwhich water can pass, a pressure adjusting device which adjusts thepressure of cooling water in the cooling water passage, and a controllerwhich functions to: determine whether warmup of the fuel cell stack iscomplete based on the running state of the fuel cell stack, and beforewarmup of the fuel cell stack is complete, control the pressureadjusting device to decrease the cooling water pressure lower than apredetermined cooling water pressure used in a steady state after warmupis complete.
 2. The fuel cell system as defined in claim 1, wherein: thecontroller further functions, before warmup of the fuel cell stack iscomplete, to control the pressure adjusting device to set the coolingwater pressure to a pressure at which gas at the electrode does notpenetrate the cooling water passage.
 3. The fuel cell system as definedin claim 1, wherein: the controller further functions, before warmup ofthe fuel cell stack is complete, to control the pressure adjustingdevice to set the cooling water pressure higher than a minimum value atwhich gas at the electrode does not penetrate the cooling water passage.4. The fuel cell system as defined in claim 3, further comprising asensor which detects a differential pressure between the cooling waterpassage and the electrode at the inlet of the fuel cell stack, and thecontroller further functions to determine the minimum value of thepressure at which gas at the electrode does not penetrate the coolingwater passage according to the measured differential pressure.
 5. Thefuel cell system as defined in claim 1 further comprising a sensor whichdetects the temperature of the fuel cell stack, and the controllerfurther functions to determine that warmup of the fuel cell stack iscomplete when the temperature of the fuel cell stack has risen to apredetermined temperature.
 6. The fuel cell system as defined in claim5, wherein: the sensor which detects the temperature of the fuel cellstack detects the internal temperature of the fuel cell stack, and thecontroller further functions to determine that warmup of the fuel cellstack is complete when the internal temperature of the fuel cell stackhas risen to a predetermined temperature at which water clogging in thefuel cell stack does not occur.
 7. The fuel cell system as defined inclaim 1, wherein: the fuel cell stack includes plural cells, the systemfurther comprises sensors which detect the temperatures of the cells,and the controller further functions to determine that warmup of thefuel cell stack is complete when the average temperature of the cellshas risen to a predetermined temperature at which water clogging in thefuel cell stack does not occur.
 8. The fuel cell system as defined inclaim 7, wherein: the controller further functions to predict waterclogging in the fuel cell stack from the cell temperature distribution,and controls the pressure of the cooling water to be higher than aminimum value at which gas at the electrode does not penetrate thecooling water passage.
 9. The fuel cell system as defined in claim 1,further comprising a sensor which detects the temperature of the fluidsupplied to the fuel cell stack at the outlet of the fuel cell stack,and the controller further functions to determine that warmup of thefuel cell stack is complete when the temperature of the fluid at theoutlet of the fuel cell stack has risen to a predetermined temperature.10. The fuel cell system as defined in claim 1, further comprising asensor which detects a temperature difference of the fluid supplied tothe fuel cell stack at the inlet and outlet of the fuel cell stack,wherein: the controller further functions to determine that warmup ofthe fuel cell stack is complete when the temperature difference of thefluid at the inlet and outlet of the fuel cell stack has increased to apredetermined temperature.
 11. The fuel cell system as defined in claim9, wherein the fluid is one of cooling water, fuel gas and air suppliedto the fuel cell stack.
 12. The fuel cell system as defined in claim 10,wherein the fluid is one of cooling water, fuel gas and air supplied tothe fuel cell stack.
 13. The fuel cell system as defined in claim 1,wherein the controller further functions to: determine thehumidification state of the membrane electrode of the fuel cell stackfrom the running state of the fuel cell stack, and when the membraneelectrode is not sufficiently humidified, the pressure of the coolingwater is increased to suppress outflow of water from the electrode tothe cooling water passage.
 14. A startup method for a fuel cell systemincluding a fuel cell stack wherein a cooling water passage and anelectrode are connected via a porous plate through which water can passand a pressure adjusting device which adjusts the pressure of coolingwater in the cooling water passage, the method comprising: determiningwhether warmup of the fuel cell stack is complete based on the runningstate of the fuel cell stack, and before warmup of the fuel cell stackis complete, controlling the pressure adjusting device to decrease thecooling water pressure lower than a predetermined cooling water pressureused in steady state after warmup is complete.